Line focus cathode structure



Nov. 5, 1935. EHRKE 2,019,600

LINE FOCUS CATHODE STRUCTURE Filed July 14. 1932 2 Sheets-Sheet 1 INVENTOR Z..F.'Ef/RKE ATTORNE NOV. 5, EHRKE LINE FOCUS CATHODE STRUCTURE Filed July 14, 1932 2 Sheets-Sheet 2 INVENTOR L FEHP/(E 277 ATT ORNE Patented Nov. 5, 1935 OFFWE LINE FOCUS CATHODE STRUCTURE Louis F. Ehrke, Newark, N. J assignor to Westinghouse Lamp Company, a corporation of Pennsylvania Application July 14, 1932, Serial No. 622,424

Claims. (01. 250-35) This invention relates to electric discharge devices and more particularly to devices of this character adapted to be employed for the production of X-rays and commonly termed X-ray 1i tubes. In its more specific aspect the invention is directed to improved means adapted to produce an improved focal spot on the anode target of a tube of this type.

Since the time that Roentgen first discovered X-rays, tubes of this character have found application in a number of different fields. Among others, some of the fields where X-ray tubes are utilized include the X-ray photography or radiographic, the therapeutic and the chemical fields.

-' The general radiographic field, which is probably the oldest and most common sphere employing X-ray tubes, includes the X-ray photography of bones, organs of and foreign elements in the human body; of solids, as for example metals 'which may contain latent defects, such as gas pockets or the like.

In radiography, it is well known that the image produced on an X-ray plate or film is caused by the variations in the degree'of transparency to X-rays of the object subjected to X-rays and is basically a shadow picture. Although X-ray tubes have been successfully employed in the general radiographic field for a long period of time, the X-ray photographic plates produced sometimes have not been entirely satisfactory in that portion of the art where exceptionally sharp definition is a prerequisite, so that positive and proper interpretation thereof may be possible.

35l This difficulty has caused considerable concern in some of the X-ray photographic fields in that the photographic plates lack the nice definition essential in those fields. That is, the finished plates generally disclose an umbra gradually fading off in intensity into a penumbra with no sharp line of demarcation between the umbra and penumbra. Plates of this character are not entirely satisfactory in some fields where it is appreciated that the ideal photographic plate is 45-one disclosing an umbra with distinct and sharp boundaries.

Heretofore, it has been proposed to obviate some of the short-comings of X-ray tubes, and especially to construct high-energy X-ray tubes 50 of the line focus type, adapted to be employed in the production of X-ray photographic plates disclosing a distinct and. sharp umbra. The latermost means proposed as a solution of this problem has not been found wholly adequate.

The inadequacy has manifested itself in at least one aspect, and is probably due mainly to the elongated circular coiled type of cathode used.

One of the important disadvantages of filaments of this character is that the band focal spot is not of uniform intensity, but is character- 5' ized by at least one, and often more than one, longitudinal very intense line located within and/or on the boundaries of the band focal spot, as shown in Figures 5 and 6, hereinafter identified.

The attainment of a focal band of non-uniform intensity or characterized by a readily apparent non-uniform distribution of electrons over the focal spot on the target, I believe has been due mainly to the configuration of filament per se. One explanation for this obviously undesirable effect is that when a circular coiled filament is employed there is a repulsive force exerted by the center or crown of the elongated filament on the electrons, emitted from those portions of 0 the filament on either side of said crown.

In order that I may avoid the above diii'iculties, I have developed the following invention hereinafter described in the specification and illus trated in the appended drawings.

Briefly, the invention comprises a novel X-ray tube embodying a new cathode assembly consisting of a focusing cup of appropriate and novel: design, associated with an electron emitting cathode also of novel configuration or design. This assembly, and especially the cathode or electron emitting filament, is of such design that even with a high energy input the electrons emitted from the cathode impinge upon the anode target in the form of a band of substantially uniform intensity.

An object of the invention is to provide an X-ray tube adapted to be effectively employed in the radiographic fields where sharp and clear definition is a prerequisite.

Another object of the invention is to provide an X-ray tube adapted to be effectively employed in the short exposure radiographic field to provide sharp and distinct X-ray photographic plates.

Another object of my invention is to provide an X-ray tube adapted to efiiciently operate under high energy inputs to produce a substantially uniform distribution of electrons on the focal area of the anode target.

Another object of my invention is to provide a novel X-ray cathode assembly adapted to provide a substantially uniform distributed band of electrons on the target of an anode.

Another object of my invention is to provide 1a. novel filamentary cathode design for an X-ray ube.

Other objects and advantages of my invention Will be readily apparent from the following description and appended drawings wherein,

Figure 1 represents a cross-sectional view with some of the parts in elevation of an X-ray tube embodying my invention;

Figure .2 is a view taken along line 11-11 of Figure 1 in the direction of the arrows;

Figure 3 is an enlarged section taken along line IIIIII of Figure 2 in the direction of the arrows;

Figure 4 is an enlarged imaginary view of the effective focal spot on the target taken along line IV-IV of Figure l in the direction of the arrows;

Figure 5 is an enlarged imaginary view of a focal spot obtained on the target of an 'X-ray tube employing a circular coiled elongated filament;

Figure 6 is another enlarged imaginary view of a focal spot obtained on the target of an X-ray tube employing a circular coiled elongated filament;

Fig. '7 is a fractional perspective view on an enlarged scale of the filament cathode in Figure 1 Figure 8 is a fractional perspective View on an enlarged scale of a cathode which is a modification of the cathode filament shown in Figure 7;

Figure 9 is a further modification of the cathode structure shown in Figure '7;

Figure 10 represents a cathode assembly and is a modification of the cathode assembly illustrated in Figure l;

Figure 11 is a still further modification of the cathode embodied in Figure 1 and is a. fractional perspective view on an enlarged scale of the filament embodied in Figure 10.

As shown in Figure 1, my novel X-ray tube comprises an evacuated gas-tight enclosing envelope, of the usual insulating material, such as glass or the like, provided with an anode I l and cathode structure l2. The anode H is supported in the usual manner by a re-entrant tube (not shown) at the anode end of the envelope, and the cathode assembly 12 is supported by a plurality of rigid conductors I3 and I4 sealed into a press f5 located at the free end of a re-entrant tube H5.

The anode H extending along the longitudinal axis of the tube, generally comprises a copper rod provided with an X-ray target I! of tungsten, tantalum or the like embedded in its free end. The face of the target I1 may be at any desired angle and, as illustrated, 20 to the longitudinal axis of the anode II.

In order that a uniform distribution of electrons over the focal spot on the target of the anode, as shown in Figure 4, may be obtained when high energy inputs are employed and so that the face of the X-ray target may be preserved without any disintegration or fusion thereof, there is employed, according to my invention, the cathode assembly l2. The cathode assembly I2 comprises essentially a focusing cup iii in which is mounted a filamentary cathode it.

In accordance with the invention, and as illustrated, the focusing cup l9 'may comprise essentially a wedge-shaped metallic block. Extending along the entire length of the block, with the slanting face of the block defining the upper limit thereof, is an outwardly flared groove 2! whose lower extremity is substantially parallel to its upper limit. I Adjacent said groove and extending into said block a distance about two or three times the depth of the filamentary cathode I8 is a V-shaped slot 22 having a rounded apex.

The upper limit of said V-slot 22 is contiguous with the lower limit of said outwardly flared 5 groove 2|. The length of said slot is defined by the parallel ends 22 of said block which are spaced .from each other a distance slightly greater than the coiled length of the filament l8 and the width of said V-slot 22 is preferably slightly 10 greater than the width of the effective face of said cathode 18.

The focusing cup i9 has a shank 23' integral therewith and extending outwardly therefrom. The shank 23 has a recess into which may be inserted a relatively heavy leading-in and support wire I 3 which is sealed to the press I5 of the re-entrant tube [6 and supported thereby.

Thusthe leading-in and supporting wire I3 serves to conduct electrical energy to and support said focusing means. The other leading-in conductor is is inserted in an insulating bushing 26 located in one end 22' of the focusing cup [9 to aid in supporting said cup and to conduct electrical energyv to the cathode 48. In this manner the ends of the filament are respectively secured to thei focusing cup l9 and the conductor l4.

As shown, the focusing cup is mounted on said supporting wires and located in a position so that the inclined face of the focusing cup and the efa0 fective face of the filament shall be substantially parallel to the face of the target [1.

Accord-ing to the invention the cathode design is such that the contour of the effective face of the coiled filament shall be such as to prevent an apparent non-uniform distribution of electrons over the focal spot. I'have found that a slightly convex, plane, or concave effective cathode surface serves this purpose. Generally, I prefer to employ a concave or plane effective surface, but 40' a slight convex surf-ace whose radius of curvature is relatively great as compared to one half the width of the slot in the focusing cup also may be employed. 7

One type of my novel cathode l8 illustrated in Figure 7 comprises a coiled filament of novel configuration and composed of tungsten, tantalu-mor other thermionic material. The coiled filament may consist of a plurality of convolutions, with each convolution defining essentially an equilaterial triangle whose apices are of rounded contour. Thus each side of the coiled filament consists of a plurality of substantially straight, spaced legs 3|, all of which are of substantially the same length and located in substantially the same flat plane.

Another novel cathode l8 contemplated by this invention is the one illustrated in Figure 8. The

filamentary cathode thereillustrated also comprises essentially a coiled filament, with each convolution thereof. having rounded ends and a pair of spaced elongated straight parallel legs 35. As shown, the adjacent parallel legs 35 of each side are of substantially the same length and located in substantially the same flat plane. 05

Still another novel type of cathode IB is illustrated in Figure 9 and is a modification of my novel cathode illustrated in Figure 7 and heretofore described. This cathode, like that of Figure 7, is also of the coiled filamentary type. The coiled filament also consists of a plurality of convolutions, with each convolution defining essentially a triangle whose apices are rounded. This cathode differs from that of Figure 7 in that one side of the filament, which may face the 16 anode, consists of a plurality of slightly inwardly curved legs 31 of the same length, and general curvilinear contour, defining a concave surface.

Any one of these cathodes heretofore described may be mounted in a suitable manner in the focusing cup in order that a line focus may be obtained on the anode. Preferably the cathode is mounted in the manner illustrated in Figures 1, 2 and 3. As shown, the cathode is placed in the V-shaped opening 22 and spaced from the sides thereof, with the face to be disposed opposite the anode being substantially parallel to the face of the anode. The cathode is so disposed within the V-shaped slot 22 that its upper face or 'elfective thermionic surface is positioned slightly below the upper end of the V opening and preferably so that the effective thermionic surface is in a plane parallel to that at the upper edge of the focusing means. The so positioned cathode is, by welding, pressure, or other convenient means, electrically secured to the supporting wire l4 and the focusing cup l9.

One other aspect of my invention, disclosed in Figures 10 and 11, comprises a focusing cup of the same general construction as that heretofore described and differing therefrom only in that a plurality of insulating bushings 40 extend through the apex portion thereof and are firmly secured thereto.

In order that a cathode may be supported at more than two points, I may construct the cathode as shown in Figure 11. The cathode l8 comprises a coiled filament having a plurality of convolutions 3! of the general type shown in Figure 7 and a plurality of spaced, closed, Y-shaped convolutions 42. The shanks of the Y-shaped convolutions are secured to said bushings by means of light wires fl sealed in said bushing. The outer extremities of the cathode are secured to the supporting wires I l and the focusing cup [9. In this manner the cathode is supported at four points to reduce sagging and prevent shorting.

By utilizing the type of cathode assemblies heretofore described, and especially a filamentary cathode, whose eifective thermionic area is non-convex, but concave or plane, it is possible to efficiently operate an X-ray tube with large inputs. Thus, by virtue of my invention, even intense focal spots are not limited to one or two millimeters in width, but may be increased appreciably above this limit range without sacrificing uniformity of intensity.

Although I have shown the focusing cup and the eifective cathode face disposed parallel to the target of the anode, it is within the purview of my invention to have both the focusing cup and the eifective anode face at an angle to the anode or target face.

In the operation of a tube of this charcter embodying my invention, I have found that the tube may be operated with comparatively large current inputs and that the focal spot on the cathode is of rectangular contour, as shown in Figure 4, l

and of substantially even intensity, without the presence of lines of variable intensities.

Although I have described my invention with particularity, it is not to be limited thereby, but only by the prior art. 10

I claim: 1

1. An X-ray tube comprising an envelope, an anode and an elongated cathode therein, said cathode being a coiled filament, with the convolutions of said filament being spaced from each 15 other in the direction of the length of said cathode and being of substantially the same length, a plurality of convolutions thereof having portions thereof defining an eifective thermionic surface of substantial length and breadth, said surface 20 being substantially non-convex.

2. An X-ray tube comprising an envelope, an anode and a cathode assembly therein, said cathode assembly including a focusing cup and an elongated cathode, said cathode being a coiled fill5 ament, with the convolutions thereof spaced from each other in the direction of the length of said cathode and being of substantially the same length, a plurality of said convolutions having parts thereof defining an effective thermionic sur- I0 face of substantial length, said surface being substantially nonconvex and facing said anode.-

3. An X-ray tube comprising an envelope, an anode and a cathode assembly therein, said cathode assembly including a focusing cup and a ll coiled filament, said filament being supported by said cup at its ends and means insulated from said cup and supported by said cup and connected to said filament intermediate its ends to prevent sagging of the same, most of the convolutions of 0 said filament being of the same length.

4. An X-ray tube comprising an envelope, an anode and a cathode assembly therein, said cathode assembly comprising a focusing cup and a coiled filament, the convolutions of said filament 5 being spaced from each other in the direction of the longitudinal axis of said cathode and being of substantially the same length, said convolutions having portions thereof that are parallel to each other and defining an effective thermionic surface '0 that is substantially fiat.

5. An X-ray tube comprising an envelope, an anode and a cathode assembly therein, said cathode comprising a coiled filament, the convolutions of said filament being spaced from each other I; and defining triangles, said convolutions being of substantially the same length, with one leg of most of said convolutions being non-convex, said legs being parallel.

LOUIS F. EHRKE. 

